In October 2008 the Marine Environmental Protection Committee (MEPC) of the International Maritime Organization (IMO) agreed on the progressive reduction of maximum sulfur content in fuel used on board commercial vessels. The agreement on the reduction in sulfur content in fuel was added to the Marine Pollution (MARPOL) Annex VI Prevention of Air Pollution from Ships. The MARPOL Annex VI sets out limits on permitted SOx and NOx emissions from ship exhausts, and also sets out other limitations and/or prohibitions on emissions of ozone depleting substances from commercial vessels. MARPOL Annex VI, also establishes emission control areas (ECA) that establish more stringent standards for SOx, NOx, and particulate matter within the ECA's. The North America ECA is 200 nautical miles from North American (US and Canada) coast including US Caribbean territories and Hawaii, but excluding waters controlled by Cuba. The current European ECA's include the North Sea and the Baltic Sea, while in port, and if a ship is considered a passenger carrier (such as a cruise ship, ferry, etc.), the ECA is defined as that distance covered when departing from one European port to another European port.
According to the MARPOL Annex VI regulations added in October 2008, the maximum sulfur content in fuel used for commercial vessels is to be reduced from 1.0% to 0.1% on Jan. 1, 2015 in ECA areas. The sulfur content in fuel used for commercial vessels is also to be reduced from 3.5% to 0.5% in non-ECA areas on Jan. 1, 2020. Additionally, the MARPOL Annex VI regulations from October 2008 allowed for the use of exhaust scrubbers to reduce sulfur oxides emissions from commercial vessels to the same levels that would be achieved by lowering sulfur content in fuel itself.
In view of the current and planned stricter emission regulations, the commercial shipping industry has a few primary options available for achieving compliance with the MARPOL Annex VI air quality standards. The vessels subject to the MARPOL Annex VI air quality standards can be converted to utilize new fuel types that would satisfy the air quality standards. For example, it may be possible to retrofit ships to burn liquefied natural gas (LNG) as an alternative to the currently used marine fuel. However, such a change would likely prove to be extremely expensive, and it is not considered to provide a satisfactory level of safety. As an alternative to converting vessels to use a different fuel type, lower sulfur content grades of conventional marine fuels may be used. However, such lower sulfur content fuels may be much more costly than the conventional, higher sulfur marine fuels. Additionally, it may be necessary to undertake various equipment retrofits, modifications, or replacements to allow suitable use with the lower sulfur fuels. Such equipment changes or replacements would further increase the cost of operation, in addition to the higher cost of the lower sulfur marine fuels. Another option for satisfying the air quality standards includes capturing, or removing, sulfur gasses from the engine exhaust to meet the required standards. Typically, exhaust scrubbers may be utilized for removing unwanted components (such as sulfur gasses) from exhaust emissions. The use of scrubbers may allow higher sulfur fuels (which may be relatively less expensive) to be used with existing engines and equipment. The scrubbers may be added to the systems downstream from the engines. As such, the use of scrubbers may provide a comparatively cost effective approach to achieving the necessary air quality standards.
Many typical scrubbers are very large in size, and, therefore, may take up valuable space on ocean-going vessels. For this reason, there has been a great deal of interest in newer scrubber technologies that may provide smaller scrubbers that demand less space. Some of the smaller scrubbers may be comparable in size to current engine silencers installed on ocean vessels. The newer scrubbers, as are typical of other designs, generally have two different modes of operation: open loop and closed loop. During open loop operation, the scrubber may take sea water from the vessel's sea water chest, and may run the sea water through the scrubber once and then discharge the sea water back into sea. The open loop operation is generally used when the vessel is 12.5 nautical miles, or further, from shore. When the vessel is closer to shore, the scrubber may be operated in the closed loop mode. During closed loop operation, a scrubber liquor is circulated through the scrubber to remove the undesired components from the exhaust emissions. As the scrubber liquor is circulated, additional alkali may be added as necessary. Solids may be removed from the circulated scrubber liquor via centrifuge operation, or other suitable operation. The scrubber may typically use the closed loop operation when the vessel is 12.5 nautical miles or closer to shore and while docked in-port.
Capturing the sulfur compounds removed from the exhaust emissions by the scrubber typically uses relatively higher pH compounds such as sodium hydroxide (NaOH). The relatively higher pH compounds may act to neutralize acid formed by the combination of sulfur dioxide and water. For example, sulfuric acid may be generated by the scrubber capturing sulfur dioxide gases and converting them to a liquid phase (sulfuric acid). NaOH, which also is commonly known as caustic soda, is used by some marine companies today as the preferred alkali in closed loop scrubber systems. However, such a scrubber system may be somewhat dangerous, as leakage or spillage of liquid NaOH can cause a major safety problem due to its highly caustic nature. Safety concerns associated with NaOH exposure may be especially pronounced passenger ships, for example, which may carry passengers who may not aware of, or trained to handle, the potential hazards. Some commercially available wet scrubber systems may typically use caustic soda (NaOH) as the alkali source. A smaller number of dry scrubber systems have also been introduced. The dry scrubber systems typically use lime or hydrated lime products as the alkali source
Scrubber systems utilizing NaOH as the alkali source generally require ship owners to dedicate additional space for scrubber installation. The required space generally results in a decrease in the usable staterooms and/or cargo space available on the vessel. The loss of stateroom and/or cargo space may become a major financial burden when considering that cabin spaces are at a premium, and cruise-liners want to make this space available for as many passenger staterooms as possible. For example, a single stateroom may yield annual revenues in the hundreds of thousands of U.S. dollars on most cruise ships.